The Mandukya Upanishad is the most concise of the Indian philosophy texts. It's considered an exhaustive overview of Advaita Vedanta, which is basis for non-dualism and has inspired many Western philosophies.
It usually comes with the Karika, which is a short commentary on it written by Swami Gaudapada, one of the famous original thinkers behind Advaita.
It's filled with great analogies for understanding non-dualism, along with several overly religious or complex descriptions that could benefit from simplification.
You can find it here - be warned, it's a challenging read.